Within this description and the attached claims, the term catalyst includes any substance which, when added to a reaction mixture, changes the rate of attainment of equilibrium in the system without itself undergoing a permanent chemical change. The term catalyst includes substances which might be changed physically during the reaction they catalyse but which could in theory be recovered chemically unchanged at the end of the reaction. The term electrocatalyst refers to an electrochemically active catalyst.
There are some applications where it is desirable for a chemical species to be able to penetrate or leave an electrode. For example, electrochemical gas sensors are well known. In one configuration, working, counter and reference electrodes are connected through a potentiostatic circuit which maintains a bias potential between the working and reference electrodes. A gas to be measured penetrates the working electrode and undergoes a catalytic reaction. For example, in the case of oxidation of NO,NO+H2O⇄NO2+2H++2e−
A reduction occurs on the counter electrode:½O2+2H++2e−⇄H2O
Similar principles apply to electrochemical sensors for measuring liquid analytes.
It is also well known to use an electrode in an electrochemical cell to generate a gas, such as chlorine, or to carry out an electrochemical reaction which consumes a gas. The necessary high reaction rates can be obtained where an electrode is permeable and a higher catalytic surface area is made available. For example, the following working electrode reaction can be used to generate chlorine:2Cl−→Cl2+2e−
On the other electrode:2H++2e−→H2 
It has been known for some time to form an electrode for electrochemical uses by coating a conductive metal with boron-doped diamond. For example, U.S. Pat. No. 6,267,866 discloses an electrode for electrochemical uses made of a conductive metal mesh coated with boron-doped diamond. Boron-doped diamond has desirable electrochemical properties, such as a wide potential window. That is to say, boron-doped diamond can be used as a catalyst at high anodic or cathodic potentials without itself undergoing degradation and without electrochemically oxidising or reducing common solvents such as water. Although boron-doped diamond is an acceptable electrocatalyst for gases such as nitric oxide and ammonia, it is a very poor electrocatalyst for water oxidation or reduction. This advantage allows selective oxidation or reduction of a gas at electrochemical potentials where competing reactions are not favoured. However, known boron-doped diamond coated electrodes include an impervious layer of boron-doped diamond overlaying a conductive metal electrode.
An aim of the present invention is to provide an electrode comprising boron-doped diamond which is permeable to a reactant or reaction product.